1. Field of the Invention
The present invention relates to a video processing apparatus and a video processing method which are suitable for application to a pixel processing in a video printer, and a video printer.
2. Description of the Related Art
Recently, a sublimation type color printer has achieved higher definition. A main picture input to such color printer has a resolution which is as high as that of a video signal, and a picture having high resolution is not frequently employed as such input picture. Specifically, although the color printer can carry out a printing operation with high resolution, the resolution of the input image data is lower. Therefore, when the color printer carries out the printing operation with high resolution, the color printer must subjects the image data to a predetermined interpolation processing to convert it into an image which can be regarded as a high-resolution image data. However, since a picture quality of an output image after the printing operation considerably depends upon a method of interpolating an image data, an interpolating method which can provide a higher picture quality has been demanded.
Since the sublimation color printer can control resolution by changing gradation in a movement direction of a thermal head, i.e., a sub-scanning direction in an analog fashion, it is necessary to interpolate an image in a direction in which heating elements of the thermal head are arranged so that the printing operation of one line amount should be carried out, i.e., in a main scanning direction. Specifically, a processing for interpolating the image so that the number of the pixels in the main scanning direction should be doubled has been demanded. Such image interpolation method includes a simple enlargement method in which a pair of the same pixels are arranged to simply double the number of the pixels, a mean value interpolation method of employing a mean value of the pixels in the vertical direction as an interpolation pixel. Though the simple enlargement method, the mean value interpolation method and so on have mainly been employed as such image interpolation method, such image interpolation method also includes a pattern recognition mean-value interpolation method which is a high-level interpolation processing employing an information about neighbor pixels and which employs a mean value of pixels in an oblique direction as an interpolation pixel depending upon a pattern of a relationship in size among neighbor pixels.
A processing changing resolution of an image by changing the number of pixels in the main scanning direction at a desired scale factor has been demanded. Such image-resolution converting method includes an interpolatory calculation of a nearest neighbor method in which an input pixel closest to an output pixel position is employed as an output pixel, an internally-dividing-point interpolatory calculation method (by-linear method) of employing a weighted mean value of four input pixels at positions close to an output pixel position as an output pixel, and so on. Such image-resolution converting method also includes a three-dimensional convolution interpolatory calculation method (by-cubic method) of employing as an output pixel a value obtained by subjecting to a convolution sixteen input pixels close to an output pixel and a sinc function (sin x/x≈x3+x2+ . . . ).
The above image interpolation methods are involved in the following problems. Specifically, when the image interpolation method employs the simple enlargement method, since pixels at edges of an image are kept, the image is prevented from being blur but has a jaggy edge in the diagonal direction, which leads to a deteriorated picture quality of the image. When the mean value interpolation method is employed, since the mean value is employed as the interpolation pixel, the image is blurred and has a slight jaggy edge in the diagonal direction. When the pattern recognition mean value interpolation method is employed, the jaggy edge is corrected by interpolation so as to become smooth, but the mean value is employed as an interpolation pixel, the image is blurred. Further, when an artificial image such as a character or the like is printed, a line, of a character, curved at a corner at right angle is converted by interpolation into a diagonal ling and hence the character having such corner is deformed.
When the interpolatory calculation method employing the nearest neighbor method is employed, since the pixels at the edge of the image is kept, the image is prevented from being blurred, but the edge in the diagonal direction becomes jaggy, which leads to a deteriorated quality of an image. When the internally-dividing-point interpolatory calculation method is employed, since the mean value is employed as an interpolation pixel, the image is blurred and the edge thereof in the diagonal direction becomes slightly jaggy. When the three-dimensional convolution interpolatory calculation method is employed, it is possible to obtain an image having a high picture quality, but, if the method is realized in the form of a hardware, then a large number of multipliers are required, which leads to increase of the manufacturing costs of such hardware. Therefore, the interpolatory calculation method of the nearest neighbor method and the internally-dividing-point interpolatory calculation method have a problem of a picture quality, and the three-dimensional convolution interpolatory calculation method has a problem of its manufacturing cost.
Moreover, in order to process an image having high picture quality, the printer has carried out a sharpness processing for enhancing an image. If the hard sharpness processing is carried out, then a boundary of a color forming an image is made sharp to thereby enhance the whole image. Contrary, if the soft sharpness processing is carried out, then the whole image becomes soft in sharpness.
Such sharpness processing includes a processing employing a quadratic differential method. The quadratic differential method is a method which, when an image data is passed through an enhancement filter, employs a value obtained by subjecting an image data to a quadratic differential as an enhancement component. For example, coefficients of the enhancement filter are set to xe2x88x920.25, 0.5 and xe2x88x920.25 and then respectively allocated to sampling points of Zxe2x88x921, 0, Z+1, and respective pixels of the image data are subjected to calculation by using these coefficients.
However, since, when a picture frequency xcfx89=xcfx80 is established, the enhancement filter employed in the above image enhancement method mainly enhances a frequency band of xcfx80/2 or higher, if the enhancement filter filters an image having a low frequency band, an image which has already been interpolated, and so on, only an aliasing component (aliasing signal) is enhanced and hence a high frequency band component inherent in the image is not enhanced much.
In view of such aspect, an image interpolating apparatus, it is a first object of the present invention is to provide an image interpolating method and a video printer which can prevent an image from being blurred and which can prevent a character and so on to be printed from being deformed.
It is a second object of the present invention is to provide an image resolution converting apparatus, an image resolution converting method and a video printer which can prevent an image from being blurred and which can convert a resolution of the image at a desired scale factor with a simple arrangement.
It is a third aspect of the present invention to provide a picture enhancement apparatus, a picture enhancement method and a video printer which can effectively enhance an image having a low frequency band.
According to a first aspect of the present invention, a video processing apparatus includes edge detecting means for detecting an edge direction of an image, coefficient selecting means for selecting a coefficient based on the edge direction detected by the edge detecting means, and filter means for filtering a frequency band by using a frequency characteristic corresponding to the coefficient selected by the coefficient selecting means, wherein the number of pixels of the image is enlarged twice.
According to a second aspect of the present invention, a video processing apparatus includes enlarging means for up-sampling video data to enlarge the number of data at a predetermined scale factor, decimating means for decimating the pixels by outputting the pixels of the enlarged video data at a desired ratio with employing an input the video data enlarged by the enlarging means. Resolution of the video data is converted at a desired scale factor.
According to a third aspect of the present invention, a picture enhancement apparatus includes first enhancement means for enhancing a first band which is a higher frequency band in a frequency band of an input video data, second enhancement means for enhancing a second band which is a lower frequency band in a frequency band thereof. A predetermined frequency band of the video data is enhanced by using the first enhancement means and the second enhancement means.